There is a constant need for technologies capable of tracking an object's position. In the field of transportation, there is a need to track the position of ships, airplanes, vehicles, etc. Military activities have always required the use of position tracking, such as the need to determine the position of a bombing target. Law enforcement agencies frequently need to determine the position of a subject individual or object, such as a package of contraband, within a search area. On a smaller scale, there is a need for technology to track precisely an object's position within a small volume. For example, three-dimensional computer design tools for animated films often map the position of one or more optical position reflectors located on selected portions of an actor's body. In the field of medicine, position tracking is frequently used, for example to locate the position of a tumor. In one example, gamma ray sources are injected into a patient and accumulate at the location of a tumor, identifying its position.
A variety of position tracking technologies have been developed to determine the location of an object of interest from a distance. At global distances GPS provides position tracking for aircraft, land vehicles, ships, hikers, explorers, etc. Radio frequency time-delay systems such as Long Range Navigation (LORAN) also provide position tracking capability, for example for ships. Radar is another tool widely used to track objects. Air traffic controllers use radar to track airplanes on the ground and in the air, NASA uses radar to track space debris, the military uses radar to detect enemy positions and to guide weapons, and meteorologists use radar to track storms, hurricanes, and tornadoes. At smaller distances, such as within spaces confined to a building or even a room, time delay sensing is also practical, as the distance between a target and receiver is still large enough to produce a measurable delay. Another technology that may be used at these distances is ultrasound. In ultrasound systems, a receiver/transmitter mounted in the tracking space, such as a room, emits a pulse and listens for an echo as it reflects back from the target.
At even smaller distances, such as for a volume of several cubic feet or less, the above-described position detection systems become currently impractical or unworkable. The operational space of such size is too small for measuring a radio frequency time delay in a practical and easily implemented manner. If a radio-frequency transmitter were mounted within such a volume, the distance between the transmitter and any object within the volume may be too small to measure a time-delay with the precision needed for position tracking in most applications. Ultrasound also fails on a small scale, for reasons similar to those described above. Various optical techniques exist to wirelessly track position within a small volume, for example, by using multiple infrared lasers; however, these systems require the target object to remain in constant view of the tracking system, thus limiting the systems' usefulness.
Another approach to position tracking at small distances is to detect fluctuations in a magnetic field/radio signal. For example, U.S. Pat. No. 6,404,340 discloses a system that uses three pairs of coils positioned in a cubical configuration. The coils generate a substantially uniform magnetic field within a defined region. The coils, when energized, produce a signal that sweeps throughout a predetermined range of frequencies. A magnetic resonance tag is placed within the field; when energized at its resonance frequency, the tag retransmits a signal which is detected by the system and is used to determine the tag's position.